New research indicates that frequently used pesticides, including types that
were once thought to be relatively benign, make be linked to the widespread
disappearance of California frog populations. A researcher at California State
University, Sacramento has found evidence that frog declines are associated
with upwind pesticide use.

Sacramento State environmental studies professor Carlos Davidson says there
is a strong association between upwind pesticide use and declines in four frog
species: the red-legged frog, the mountain yellow-legged frog, the foothill
yellow-legged frog and the Cascades frog. And the declines were most strongly
associated with the use of cholinesterase-inhibiting pesticides, which include
many of today’s most heavily used pesticides. Davidson’s findings
appear in the December issue of the Journal of Ecological Applications.

The study builds on previously published findings suggesting historic applications
of wind-borne agrochemicals as a factor in the declines.

“If it turns out organophosphate and carbamate pesticides are the cause,
it will be a cautionary tale about the use of new chemicals,” Davidson
says. “When DDT and other organochlorine pesticides were phased out because
they were harming wildlife species like birds, farmers switched to a new generation
of pesticide. Now, 30 years later, we’re finding those pesticides may
be harming another wildlife group. It shows it may take a long time to figure
out the environmental impact of new chemicals.”

Davidson mapped out the disappearance of five species of California frogs and
examined the association between the spatial patterns of decline and the historic
pattern of pesticide use in California from 1974 to 1991. Pesticide application
data came from the Department of Pesticide Regulation records.

Davidson’s
study included more than a thousand historic frog sites covering almost the
entire state. At some sites, frogs were still present while at others the frogs
have now disappeared. At each site, Davidson calculated the predominant wind
direction and the amount of pesticide use upwind from the sites. Across the
five species upwind pesticide use at sites where frogs had disappeared ranged
from two to 12 times greater than for sites where the frogs still exist. Of
the five species studied, only the Yosemite toad did not show declines associated
with pesticide use.

The study also points to the need for further research because although Davidson
found evidence that pesticides are contributing to amphibian population declines,
the likely pesticide levels at most sites are well below concentrations found
to be lethal in the laboratory. He says this suggests that pesticides are having
sub-lethal effects and interacting with other factors.

Davidson can be reached at (916) 278-6063. Media assistance is available by
calling the Sacramento State public affairs office at (916) 278-6156.